This invention relates to a process for product obtained by hydrophobizing highly dispersed oxides, mixed oxides and oxide mixtures of metals and/or metalloids by treating the oxide particles with volatilizable organosilicon compounds in the gas phase.
It is known to hydrophobize highly dispersed oxides (active fillers) which have been obtained by reaction of metal or metalloid compounds or volatile compounds thereof in vapor form at elevated temperatures in the presence of a hydrolyzing and possibly also an oxidizing agent. The oxides in this process are rendered hydrophobic by reaction with a halogen containing inorganic or organic silicon compound.
Oxide-aerogels usually are made by subjecting volatile compounds of metal or metalloids, particularly the halides or gas mixtures containing the same, in the gaseous phase to the hydrolyzing influence of water vapor, whereby the resulting oxides, present in the aerosol state, form aerogels and then isolating these products from the easily condensed, gaseous reaction products at a temperature above their dew points. The water vapor forming gas mixtures can consist of combustible, particularly hydrogen containing gas mixtures or compositions which form such mixtures and also of non-combustible, preferably oxygen containing gases. The oxides, obtained have a primary particle size of less than 150m.mu.. As starting materials for this process, there may be used volatile halides and preferably chlorides and fluorides.
In the preparation of mixed oxides, different metals or metalloids or compounds thereof which are volatile are introduced simultaneously as gaseous mixtures into the thermal reaction, so as to cause the oxides to separate out in the form of mixed oxides. The preparation, on the other hand, of so-called oxide mixtures is effected by separately subjecting different volatile reaction compounds to the pyrolytic treatment but jointly converting the separate materials from the aerosol to the aerogel state, i.e., co-coagulating them, so that the obtained oxides are in the form of oxide mixtures. It is also possible to subject different and separate oxides after their preparation to a mechanical treatment for combining them to form oxide mixtures.
If, in the thermal reaction, halogen containing starting materials, for example, silicontetrachloride or silicontetrafluoride are used, there are obtained products which, as a result of their high adsorption capacities, contain large amounts of hydrogenhalide and also contain halogen directly bound to the metal or metalloid atom. These oxides are strongly acid in their reactions. Their contents of hydrohalic acid may, for example, amount to 0.1%, so that they have a pH value of about 1.8. These oxides exhibit predominantly hydrophilic properties.
For many purposes, for instance, for working highly dispersed filler materials into organic media, it is desirable that the filler material possess organophilic, that is, hydrophobic properties, for which organo-chlorosilanes and various other agents have been used in various processes. Thus, it is known to hydrophobize pulverulent silicic acid through treatment with alkylchlorosilanes so as to form a coating thereon. In this case, however, the chlorosilane present on the surface of the silicic acid adsorbs water giving rise to the formation of hydrochloric acid. The thus hydrophobized silicic acid must be freed from the formed hydrochloric acid.
Hydrophobizing of powdery silicic acid with silicone oils has also been proposed. This involves suspending the dry pulverulent silicic acid in an organic liquid.
Pyrogenic metal oxides which on their surface have free OH groups also have been treated with gaseous or readily vaporizable materials, such as alcohols or formaldehyde and ketenes, the oxides undergoing etherification, esterification or acetate formation. This treatment has been carried out following or simultaneously with a hydrolysis with water or steam. In the esterification there are obtained, similar to the relatively unstable products obtained in saponification, products which in general do not meet the requirements for stability in hydrophobic products. The esterification-modified products have therefore not achieved industrial importance as truly stable hydrophobic products.
It is furthermore known to treat highly dispersed oxides by hydrophobizing them with silanes in vapor form whereby the hydrophobizing agent is added directly after the formation of the oxide from the halide in the presence of steam and oxygen at a temperature under 500.degree. C. The hydrophobizing takes place in the presence of free hydrogen halide formed in the production of the oxides, the hydrogen halide being present in large amounts. The resulting products do not have a pH value exceeding 2.0.
In the forementioned procedures, chemical reactions with the OH-groups on the oxide's surface do not take place, but the reaction rather is only with the surface adsorbed water, so that fine particle oxides in a stable form are not obtained. A stable hydrophobized material can be obtained only when a chemical reaction is involved. Only highly dispersed oxides hydrophobized through a true chemical reaction do not undergo extraction, e.g. from carbon tetrachloride by shaking with water. The other products which are not formed by chemical reaction with the OH-groups are extracted into the aqueous phase, since by means of the carbontetrachloride the merely adsorbed organic molecule is dissolved off its surface.
Attempts for altering the properties of a precipitated metal or metalloid oxide by hydrophobizing the same through reaction of the OH-groups present on the surface thereof have not been lacking.
Thus in German Patent No. 1,163,784, a process is described for the surface treatment of highly dispersed metal and/or metalloid oxides which may be homogeneous oxides, or mechanical mixtures, or mixed oxides or oxide mixtures and which have free OH-groups on their surface. The oxides are obtained by thermal decomposition of volatile compounds of these metal and/or metalloid compounds in vapor form in the presence of hydrolyzing and/or oxidizing gases or vapors. They are treated in uncondensed form obtained freshly from the place of their formation. Prior to the hydrophobizing treatment they are freed as far as possible of halogen, hydrogen halide, and adsorptively bound water under exclusion of oxygen. The oxides are then homogeneously mixed with hydrophobizing substances capable of reacting with the OH-groups. For this purpose they are introduced together with small amounts of steam and advantageously with an inert carrier gas into a continuously operated concurrent flow reactor in the form of a vertical tubular oven. The reaction chamber is heated to a temperature of 200.degree. to 800.degree. C and preferably to from 400.degree. to 600.degree. C. The resulting solid and gaseous reaction products are finally separated and the solid products are preferably deacidified and dried. Contact with oxygen is not effected till after cooling to below about 200.degree. C.
The surface treatment with the compounds adapted for reaction with the OH-groups must take place in the presence of small amounts of steam with the result that the thermally destroyed groups are reformed. It is recommended that for 100 m.sup.2 surface area of the oxide about 0.5 to 2.0 m mol water be introduced. The treating agent for the reaction is introduced in an amount dependent on the surface area and the ultimate application. A highly dispersed silicic acid having a surface area of 200 m.sup.2 /g has about 1 m mol/g free OH-groups. This would indicate that theoretically 1 mol/g of reagent should be introduced for reacting therewith. However, in practice, it is advantageous to use 1.5 m mol/g thereof.
As reactants for the surface treatment, there may be used in accordance with prior art process, any compounds which will react with OH-groups, as for instance, by etherification, esterification or acetal formation.
Suitable reactants include alcohols, aldehydes, ketenes, alkylene oxides and the like. Particularly good results are obtained if the oxide is reacted with the corresponding halide of the treatment compound. The finished oxides possess organophilic properties and can be dispersed in organic media, as for instance lacquers with advantageous results.
In order to obtain hydrophobic properties, there can be used the known hydrophobizing agents, preferably alkyl or aryl or mixed alkyl-aryl halogenosilanes and most preferably dimethyldichlorosilane, or also the corresponding esters of the silanes. The latter do not produce optimally stable products, but have the advantage that in their used hydrogen halide is not split off, thus eliminating the necessity for deacidification.
The organophilic or hydrophobic fillers produced by the aforesaid processes find many uses, for instance as free-flowing agents in powder systems, as fillers in special coating compounds, e.g. paint primers, as fillers for plastics and elastomers such as natural and synthetic rubber.
However, fillers for use in silicone rubber have to meet additional requirements, such as being halogen-free and having a greater thickening effect than the above mentioned hydrophobic products. These fillers are therefore preferably formed by treatment of the oxides with organosiloxanes. For this purpose, a number of processes have become known for "coating" natural and synthetic fillers, for instance silicic acid or materials containing the same. In this connection, the finely divided filler is mixed with the liquid siloxane or treated in a fluidizing bed with finely dispersed siloxane whereby more or less strongly adhering coatings on the filler surface are obtained. In order to obtain the optimum degree of adhesiveness between the hydrophobizing agent and the filler particles, it is necessary that there be a chemical bond between the two. The prior art processes have not been acceptable because of apparatus limitations or the time required for adequate mixing of the components.
To economically carry out the reaction of, for example, pyrogenically produced silicic acid with siloxanes, such as D.sub.4 octamethylcyclotetrasiloxane, use is made in a prior art process (British Patent No. 932,753, U.S. Pat. No. 2,803,617) of an acid or like material as catalyst for the reaction.
According to another known procedure, the reaction is carried out without pressure but the treatment of the silicic acid is effected in batches and involves extended residence times, e.g. 3-4 hours in stages of the process. Thus a continuous process is hardly possible in an economical manner.
The object of the invention therefor is to provide highly dispersed oxidic fillers in an economically and technically feasible manner, which fillers are distinguished by their stability and optimal hydrophobic and organophilic properties and thereafter are particularly suitable for use as additives in silicone rubbers.